It is sometimes desirable to move an automobile or other vehicle without using its engine and driving the vehicle in the usual manner. For example, the vehicle may be inoperative and therefore unable to be driven, necessitating that it be towed or pulled to a repair shop. Properly operating vehicles are also sometimes towed. Many owners of recreational vehicles or mobile homes will pull a car behind the mobile home as they travel so that they do not have to rely on the mobile home for their only source of transportation. Furthermore, if it is necessary to transport two vehicles but only one driver is available, one vehicle is often used to tow the second vehicle.
In most vehicles, the wheels that drive the vehicle (e.g., the front wheels on a front-wheel drive vehicle, the rear wheels on a rear-wheel drive vehicle, and every wheel on a four-wheel drive car) are directly connected to the transmission through the vehicle's drive shaft. Therefore, if the vehicle's wheels are rotated when the vehicle is being towed (which would happen, e.g., if the vehicle was pulled), the rotation of the wheels will cause components within the transmission to rotate, and heat will be generated by the friction caused by these moving components. If the vehicle is towed over a substantial distance, the amount of heat generated can seriously damage or destroy the transmission.
In order to avoid this problem, cars are usually towed by raising the appropriate wheels above the ground (which would be all four wheels on a four-wheel drive vehicle) so that the wheels do not turn as the car is moved. People who want to tow their car behind a recreational vehicle, for example, will usually rent a trailer and drive the car to be towed up onto the trailer so that the car can be pulled on the trailer without rotating any of the car's wheels.